Originally Posted by neurologicablog
The spinning girl is a form of the more general spinning silhouette illusion.
The image is not objectively spinning in one direction or the other.
It is a two-dimensional image that is simply shifting back and forth.
But our brains did not evolve to interpret two-dimensional representations of the world but the actual three-dimensional world.
So our visual processing assumes we are looking at a 3-D image and it uses clues to interpret it as such.
Or, without adequate clues it may just arbitrarily decide a best fit spinning clockwise or counter-clockwise.
And once this fit is chosen, the illusion is complete we see a 3-D spinning image.
By looking (somewhere else) around the image, focusing on the shadow or some other part, you may force your visual system to reconstruct the image and it may choose the opposite direction, and suddenly the image will spin in the opposite direction.